Duripan Horizon and Durinodes - Soil Geography of the USA: A Diagnostic-Horizon Approach - page 129

Chapter 14
Duripan Horizon and Durinodes

14.1 Introduction

Soils containing horizons cemented with silica occur worldwide, especially in the
subhumid, Mediterranean, and semiarid regions. When the subsoil is cemented by silica
into a hardpan, it is referred to as a “duripan” or in the geological literature as “silcrete”
or “duricrust” (Milnes and Twidale 1983 ). In Soil Taxonomy (Soil Survey Staff 2010 ), a
duripan is defined as a “silica-cemented subsurface horizon with or without auxiliary
cementing agents.” Other cementing agents could be Fe oxides and CaCO 3 .The
duripan horizon can occur concurrently with a petrocalcic horizon (Chadwick
et al. 1987 ; Boettinger and Southard 1991 ; Blank and Fosberg 1991 ; Eghbal and
Southard 1993a , b ;Blanketal. 1998 ; Hobson and Dahlgren 1998 ); the duripan horizon
often underlies an argillic or cambic horizon (Flach et al. 1969 ; Torrent et al. 1980 ;
Chadwick et al. 1987 ; Blank and Fosberg 1991 ; Munk and Southard 1993 ;Blank
et al. 1998 ; Hobson and Dahlgren 1998 ). A key aspect of the duripan is that it slakes
in KOH or NaOH but does not readily slake in 1 M HCl (Soil Survey Staff 2010 ).
Durinodes are “weakly cemented to indurated nodules or concretions with
a diameter of 1 cm or more” (Soil Survey Staff 2010 ). Durinodes do not slake in water
but break down in hot concentrated KOH after treatment with HCl to remove
carbonates. For both duripan and durinodes, the cement is primarily opal-A and
other microcrystalline forms of silica. Most of the silica cements are derived from
weathering of siliceous minerals (Torrent et al. 1980 ;BoettingerandSouthard 1991 ;
Munk and Southard 1993 ; Hobson and Dahlgren 1998 ; Kendrick and Graham 2004 ;
White et al. 2005 ), volcanic ash (Flach et al. 1969 ; Blank et al. 1998 ;Chadwick
et al. 1989 ), or siliceous loess (Blank and Fosberg 1991 ; Othberg et al. 1997 ).
Soils with duripans have been reported in many parts of the world, including
Australia (Milnes and Twidale 1983 ; Chartres 1985 ; Chartres and Fitzgerald 1990 ;
Chartres and Norton 1993 ), Africa (Smale 1973 ; Smith and Whalley 1982 ), Mexico
(Flores-Rom´n et al. 1996 ), and the USA. According to Flach et al. ( 1974 ),
soils with duripans also occur along the Pacific Coast of Central America and in
Italy and Greece. In the USA, duripans have been studied in southwestern ID